Multi-layered composite materials have been used in orthopaedic fixation devices. Thermoplastic and thermoset fiber-reinforced composite materials have been used to reinforce, repair or even replace metallic structures. When used in orthopaedic devices, composite structures should be monitored independently from their domain of use (e.g., the surrounding bone or other tissue), especially for load-bearing structures to avoid catastrophic failures resulting from structural cracking, delamination, and de-bonding of the fibers from the matrix. High-strength composite materials are often manufactured in the form of tapes or sheets, otherwise known as pre-impregnated materials or “pre-preg,” and accordingly, generally require sensors or “smart strips” that are embedded within the structure, depending upon the structural layer that has to be monitored. In the case of cannulated orthopaedic implants, these sensors are generally placed as far as possible from the neutral axis to ensure adequate sensitivity to applied forces and moments.
Embedded sensors in structures such as implants have generally decreased the mechanical properties of the composite material due to the dimensions of the sensor and wireless electronics. Machining pockets in the pre-preg tape after the pre-preg tape is wound around a mandrel also have led to an increase in the number of discontinuous fiber ends resulting in local stress risers and increased risk of an adverse biological reaction. The use of fiber optic sensors in composite structures to monitor the cure of composite materials in real time during manufacture, and to monitor the in-service structural health of composite structures have also required expensive signal processing and interrogation technology and, because of their large size, have prohibited them from being embedded into composite structures without having deleterious effects on their physical properties. In addition, fiber optic sensors have tended to induce local stress in the laminates when subjected to mechanical loads and environmental changes such as temperature and moisture making the interfaces susceptible to de-bonding, and they have exhibited poor handling characteristics, i.e., they are unlikely to survive the high temperatures and pressures associated with composite processing.
Moreover, because a strain measuring element is required to be in permanent mechanical coupling with the host structure in order to effectively transfer strain with no losses due to sliding or adhesive failure, attempts using thin film gauging methods have led to thermal damage to the polymer composite structure due to the high temperature required during the sputtering process.
Bonding to the host substrate can be achieved by integrating the strain gauge unit within the pre-preg tape in a similar manner to the reinforcing glass or carbon fibers. Integration of a fiber optic sensor directly into the main composite component, i.e. into the composite tape is another option but these devices require expensive signal processing and interrogation technology. Additionally, they are delicate structures making them less suitable for orthopaedic application.